I used to be MD of a will-writing company.
Firstly, the solicitor seems determined to be an executor (and a powerful one at that, from what you've written!). There is absolutely no need to name the solicitor as executor and I'd actually advise against it. If the will is straightforward the 'non-professional' executors can obtain probate and distribute the estate without any professional assistance. If it's not, they can appoint their own solicitor anyway. (He/she can still be paid from the estate before distribution of whatever remains).
There is no reason to avoid percentages. (My own will uses them exclusively). They're probably far more practical than leaving specific legacies.
Any well-drafted will should ALWAYS state clearly what should happen if a beneficiary should pre-decease the testator. While Ubasses states what should happen if no such provisions are included in the will, the testator is free to determine to whom the money should be left. (e.g: "I give 20% of the the value of my estate, after the payment of just debts and expenses, to Helen Lisa Smith of 24 Railway Cuttings, Watton in the County of Norfolk. If the said Helen Lisa Smith shall pre-decease me, or die within 28 days of my death, I give such sum to Battersea Dogs and Cats Home at 4 Battersea Park Road, London SW8").
In general, the solicitor seems to be talking total rubbish. (If anyone needs their mental capacity tested, it would seem to be him!). However I do agree that the use of percentages should exclude any residual clause because all debts due to the estate form part of that estate, irrespective of the date upon which they're paid. (So any 'late' money should still be distributed in accordance with the percentages stated within the will).
My advice is the same as that given by Murraymints: Get another solicitor!!!
Chris